Conventionally, when images are formed with copying machines, laser beam printers, etc., the Carlson Method has been generally used (U.S. Pat. Nos. 2,221,776, 2,297,691 and 2,357,809, "Electrophotography," p22-p41, R. M. Shaffert, 1965, The Focal Press).
FIG. 2 shows a schematic view of an apparatus used in a conventional method of forming fixed images. In the conventional method, after an electrostatic latent image formed on a photoconductor by optical means is developed in a developing process, it is transferred to a recording medium such as a recording paper in a transfer process and then fixed into the final image generally with heat and pressure in a fixing process. As the photoconductor is repeatedly used, a cleaning device is provided for cleaning the residual toner after the transfer process with its rotation.
In the conventional method of forming fixed images, however, the processes from the formation of the electrostatic latent image up to its fixing onto the recording medium are time consuming, which makes the apparatus used therein not only complicated but also large. In addition, since the transfer efficiency of the toner is poor in the transfer process, it poses such problems as extra labor needed for the disposal of the toner collected by cleaning the residual toner, and pollution due to the scattering of the toner in and out of the apparatus.
Therefore, a method of simultaneously conducting transferring and fixing has been proposed (U.S. Pat. No. 4,448,872). In this method, since the transferring and fixing are simultaneously carried out by pressing the toner image developed on the dielectric drum to the recording medium, the simplification of the apparatus can be surely achieved. However, since only pressure is applied at fixing, the fixing ability is poor, and little improvement is achieved in transfer efficiency.
The fixing of the toner should be generally conducted at a high temperature due to the high melting temperature of the toner, thereby requiring an apparatus with high thermal efficiency. The fixing process usually works independently, and is carried out at such a high temperature of around 200.degree. C. Accordingly, expensive heat-resistant materials such as heat-resistant resins, heat-resistant rubbers, etc. have to be provided in the periphery of the fixing device.
In addition, when the fixing is carried out at a high temperature, it is subject to problems such as curling and jamming of the paper, etc. Therefore, taking into consideration of the radiation from the apparatus, a device highly capable of radiating heat is in demand. Further, if the fixing requires a high temperature, it takes more time to reach the set temperature so that a quick printing becomes impossible. In such a case, therefore, this method is unsuitable for devices such as a facsimile which requires quick printings.
Further, in view of solving the problems, there has been proposed a method of forming fixed images, wherein the transfer and fixing process is simultaneously carried out by adhering the toner onto a transfer film, which rotates while keeping it partly in close contact with a photoconductor to form a toner image, and putting the recording paper and the transfer film between a pressure roller and a heat roller provided away from the photoconductor (Japanese Patent Laid-Open No. 197884/1990).
According to this method, however, when the transfer film is wound around the pressure roller, one of the pair of rollers for transfer and fixing, and a heat roller is arranged on the outside of the transfer film, the recording medium, which is heated from the reverse side, such as paper, etc. having insulating effects has a poor thermal efficiency, and thereby a sufficient heat required for fixing cannot be supplied to the toner. Therefore, problems arise in that fixing becomes insufficient. On the other hand, when the transfer film is wound around the heat roller and the pressure roller is arranged on the outside of the transfer film, the heating material is arranged in the inside of the film belt, causing problems in radiation from the internal portion of the film belt. When the heating material is arranged inside the film belt, the radiation conditions are likely to be insufficient, thereby causing deterioration in sensitivity and decrease in durability of the photoconductor due to heat. In addition, since the fixing is carried out through the film, problems may arise in the delaying of the transmission of heat, thereby presumably demanding a higher fixing temperature for the heat roller.
Further, in the conventional method of forming fixed images, however, through the processes from the formation of the electrostatic latent image up to its fixing onto the recording medium, the temperature of the heating material of the fixing device has to remain at a very high level (usually around 200.degree. C.) and further a relatively high pressure is required (usually between 2.0 and 6.0 kg/cm). On the other hand, since both the photoconductor and the developing device have to be maintained at around room temperature, a considerable distance has to be maintained between the fixing device and the developing device, which necessitates to make the machine larger. In addition, it is necessary to force the removal of the generated heat from the system, but the noise produced by the forced radiation device is not negligible.
As for solving these problems, a device for carrying out low temperature fixing using a cold pressing method (Japanese Patent Laid-Open No. 159174/1984) is known. In this reference, however, although the fixing temperature is low, the nip pressure has to be elevated normally to not less than 4 kg/cm in this method, making the machine heavier. Moreover, it poses problems in the gloss of the images, deformation of the paper copy sheets and insufficient fixing strength. As for a fixing device for fixing images at such a low nip pressure of less than 4 kg/cm, a heat roller method is known, for example, but it has been pointed out that the fixing temperature needs to be maintained at not less than 120.degree. C.
Under these circumstances, the development of a fixing device that can fix images at a low temperature and at a low nip pressure is highly desired, but it has not yet been developed. Further, as regards toners to be indispensably used for the image formation, since they have been confined to those made from a thermoplastic resin dispersed with additives such as coloring agents, charge control agents, releasing agents, etc., and pulverized, there have been limitations on the molecular weight, the softening point of the thermoplastic resin for use in the toner from the aspect of storage stability, thereby posing limitations on the further pursuit of low temperature fixing.
From these standpoints, the development of a novel method of forming fixed images as well as a matching toner therefor is in demand.